This invention relates to a processing communication satellites and more specifically relates to downlink reception and transmission techniques for such satellites.
A multibeam processing satellite system requires a comprehensive and consistent approach to its transmission system, including both its uplink and its downlink. Since the uplink and downlink transmission systems are decoupled by the processing (by demodulation, decoding, and switching) on the satellite, these two parts may be defined separately. The downlink transmission system definition must include all aspects relating to the spatial, polarization, spectral, and temporal attributes and, in particular, methods of multiplexing to permit many users to be concurrently serviced by the downlink. Additionally, various data handling issues, including error control and protocol aspects relating to the way that transmission data units are organized must be defined.
A preferred form of the present invention is useful in a processing satellite communication system for generating and transmitting data in an available spectrum of frequencies suitable for use by a downlink transmitting system of a processing satellite. In such a system, a terminal is connected to receive data cells for transmission. First and second frame type signals are generated, preferably by a scheduler. A predetermined number of the data cells is grouped with a pre-determined error correction code to generate frame bodies. A first type of frame bodies are generated in response to the first frame type signal and a second type of frame bodies are generated in response to the second frame type signal, preferably by one or more error control encoders and an interleaver. The frame bodies are grouped with the header symbols and with trailer symbols to generate data frames comprising a predetermined number of symbols. The grouping is preferably done by a frame organizer. The data frames are modulated by a predetermined form of modulation to enable placement of the modulated data frames into a plurality of frequency bands having a predetermined frequency range and having a predetermined transmission rate definable in megasymbols per second. The modulating is preferably done by one or more modulators. The modulated data frames are transmitted over one or more beams with a predetermined form of polarization, preferably by a radio transmitter. Power is reduced in response to the second frame type signal.
Another preferred form of the invention is suitable for use in a processing satellite communicating system for receiving and processing radio carrier signals in an available spectrum of frequencies transmittable by a processing satellite in a downlink. In this embodiment, one or more beams of the radio carrier signals having one or more forms of polarization are received, preferably by an antenna system. The radio carrier signals are separated into a plurality of frequency bands having a predetermined frequency range and having a predetermined transmission rate definable in megasymbols per second. The frequency bands are demodulated into data frames comprising a frame body and a header including a frame type. The demodulating preferably is performed by demodulator circuits. The frame type is decoded into a first frame type and a second frame type and the frame bodies are decoded into a pre-determined number of data cells in response to the first frame type. The decoding preferably is accomplished by decoder circuitry. The decoded data cells are transmitted for further processing, preferably along a data bus. A first number of the data cells is transmitted in response to the first frame type, and a second number of data cells less than the first number is transmitted in response to the second frame type.
Still another preferred form of the invention involves generating radio waves for use in the down link of a processing communication satellite. Radio waves are generated in one or more beams having one or more forms of polarization and a predetermined form of modulation. A first group of the beams is limited to a single form of polarization.
The radio waves also are generated to represent a plurality of frequency bands having a predetermined frequency range and having a predetermined transmission rate definable in megasymbols per second. The ratio of the predetermined frequency range to the predetermined transmission rate is in the range of 1.2 to 1.3. The radio waves also are generated to represent serial TDM streams of data frames, and at least some of the streams are used with the first group of beams. The data frames comprise a predetermined number of symbols defining frame bodies with trailer symbols and with header symbols defining:
(1) frame marker symbols for delineating the data frames and for resolving modulation ambiguity,
(2) non-repeating frame number symbols indicating frame numbers of the data frames which do not repeat over the expected life of said satellite, and
(3) frame type symbols indicating different types of the data frames.
The frame bodies comprise a plurality of blocks formed by a plurality of cells which are encoded by an outer code to yield a coded block. One or more of these coded blocks are entered into a rectangular interleaver array. The interleaver array""s contents are accessed in orthogonal order to that in which it was entered and encoded a second time by an inner code and partitioned into two separate independently decodable quadrature components modulated by square-root raised cosine pulse shaping on quaternary phase shift keying. The radio waves also are generated to represent a predetermined number of data cells with a predetermined error correction code grouped within the data frames.
The foregoing techniques offer a number of advantages. For example, the techniques permit extensive frequency reuse, as well as a single access downlink which reduces intermodulation effects. The frequency placement and spectral shaping aspects of the preferred form of the invention permit efficient usage of downlink spectrum. The invention also enables null cell insertion to round out frames, thereby permitting rate decoupling. Traffic cells can be released at the earliest opportunity without explicit downlink scheduling, and any traffic rate from essentially zero up to the maximum (i.e., the downlink symbol rate) can be directed to a specific user terminal. The downlink symbol rate used in the preferred embodiment provides a well-defined reference frequency to be used as a stable reference by all ground terminals. The preferred embodiment also provides a built in reference for ambiguity resolution of the received quatenary phase downlink signals. The preferred embodiment also provides a built-in reference for precise time-of-arrival measurement of downlink frames. The preferred embodiment also provides a non-repeating downlink frame number (over the life of the system) for simple derivation of uplink synchronization. The preferred embodiment also provides for gating off of the downlink transmission when an empty frame (i.e., all null cells) would be created otherwise, thereby reducing the draw on satellite bus power. The error control methods provided by the preferred embodiment enable highly efficient usage of downlink power. The preferred embodiment enables two levels of transmission (alternatively up to eight) to accommodate adaptation to rain events and to increase usable downlink capacity. The preferred embodiment provides for identically sized frames for heavy, light (alternatively up to 8 different inner coding rates) and empty frames for simple downlink synchronization. The invention is compatible with ATM (asynchronous transfer mode) standards and provides virtually all supervisory functions via standard ATM cells. The preferred embodiment is well adapted to highly efficient DAMA methods for low duty cycle burst usage such as xe2x80x9cweb browsing.xe2x80x9d